Distributing information optically has some significant advantages over sending that same information electrically. Generally, optical signals move much faster than electrical signals and thus are characterized by shorter delays in communicating information from one location in a circuit to another. More importantly, they are capable of carrying more information than are electrical signals. And they are not susceptible to deterioration because of the parasitic capacitances and inductances that are currently limiting the attempts to increase speeds and performance in today's complex semiconductor integrated circuits. As a consequence, in recent years there have been many advances in the fields of optical communications and optical signal processing and in the optical media and the optical devices that are used in those fields.
This interest in optical signaling has also had its impact on the integrated circuit fabrication industry as more companies search for ways to combine or integrate photonic elements with the microelectronic devices that have been fabricated on IC chips. There have been many recent advances involving the fabrication of optical waveguide structures on silicon substrates, the fabrication of photodetectors to convert the light to electrical signals that can be used by conventional microelectronic circuitry and the fabrication of light emitters or laser elements for converting the electrical signals to optical signals.
In general, to build an optical signal distribution network for purposes of on-chip optical signaling, one needs to be able to make good optical waveguides to distribute the optical signals and one needs to be able to fabricate elements that get the optical signals into and out of the waveguides to interface with other circuitry. Semiconductor waveguides are being developed to guide the optical signals in directions parallel to the wafer surface. Other elements are also being developed to extract the signals from the waveguide so that they may be delivered to other circuit elements located in another layer that is either above or below the optical signal distribution network. In general, extracting the signals can be accomplished in one of two ways. Either the optical signal itself is extracted out of the waveguide and delivered to other circuitry that can convert it to the required electrical form. Or the optical signal is first converted into an electrical signal and the electrical signal is delivered to other circuitry. Extracting the optical signal as an optical signal involves the use of mirrors within the waveguides or elements that function like mirrors. The scientific literature has an increasing number of examples of technologies that can be used to construct such mirrors.